Method for producing battery separator sheet

ABSTRACT

A plastic material is formed into a sheet suitable for conversion into a battery separator. An extruded sheet is formed by calendering to produce projections on at least one surface and thereafter its thickness is made more uniform. The method preferably includes transporting the calendered sheet on the calendering roll to partially harden it. The sheet is removed from the calendering roll by withdrawing the projections substantially straight out of the depressions in the calendering roll. The plastic material is preferably oil filled. By another aspect, projections are present on opposite surfaces of the sheet and secondary molding immediately follows the initial calender molding.

Umted States Patent 1191 Hollenbeck *Nov. 4, 1975 [5 METHOD FOR PRODUCING BATTERY 2,889,390 6/1959 Schwartz 264/175' SEPARATOR SHEET 3,089,191 5/1963 Conrad 264/175 3,351,495 11/1967 Larsen et a1... 260/28.5 A Inventorl Bonderinko Hollenbeck, Pelham, 3,551,544 12/1970 Hlinka 264/175 [73] Assignee: W. R. Grace & Co., Cambridge, Pr m ry Examiner-Jeffery R. Thu'rlow M Attorney, Agent, or FirmEdward J. Hanson, Jr.; C.

E. P k 221 Filed: Feb. 4, 1974 er [*1 Notice: The portion of the term of this patent subsequent to Mar. 19, 1991 [57] ABSTRACT has been disclaimed. A plastic material is formed into a sheet suitable for [21] Appl' 439517 conversion into a battery separator. An extruded sheet Related US. Application Data is formed by calendering to produce projections on at [63] continuatiomimpart of s 225,205, p 10, least one surface and thereafter its thickness is made 1972, Pat. No. 3,798,294. more uniform. The method preferably includes transporting the calendered sheet on the calendering roll to [52] US. Cl. 264/41; 136/146; 264/49; partially harden it. The sheet is removed from the cal- 264/175; 264/210 R; 264/284 endering roll by withdrawing the projections substan- [51] Int. CI. B29D 7/06; B29D 27/00 tially straight out of the depressions in the calendering [58] Field of Search 264/41, 49, 175, 284, 211, roll. The plastic material is preferably oil filled. By an- 264/210 R; 136/146 other aspect, projections are present on opposite surfaces of the sheet and secondary molding immediately [56] References Cited follows the initial calender molding.

UNITED STATES PATENTS 2,274,260 2/1942 Schelhammer et al. 264/175 7 Claims, 6 Drawing Figures US. Patent Nov. 4, 1975 3,917,772

FIG. I

FIG. 2

FIG. 5

FIG. 6

METHOD FOR PRODUCING BATTERY SEPARATOR SHEET BACKGROUND OF THE INVENTION This application is a continuation-in-part of my copending application U.S. Ser. No. 225,205 filed Feb. 10, 1972, now U.S. Pat. No. 3,798,294.

This invention relates to a method of forming sheets suitable for conversion into. battery separators. More specifically, this invention relates to forming such sheets from plastic materials by calendering.

Battery separators have been formed in the past from the preferred plastic material of the present invention, which material is described in U.S. Pat. No. 3,351,495 owned by the same assignee as the present invention. U.S. Pat. No. 3,351,495 shows the plastic material being formed into smooth surfaced battery separators. The plastic material of U.S. Pat. No. 3,351,495 also, in the past, has been extruded through a die having a ribbed configuration to produce a sheet suitable for conversion into a ribbed battery separator.

It is an object of the present invention to provide a method for producing a very uniform sheet of material suitable for conversion into a battery separator.

It is a further object of the invention to provide such a method that will produce a sheet having extremely uniform thickness dimensions so as to enable the use of the thinnest possible separator comensurate with the necessary separation ofthe battery plates and provision of adequate channels for the escape of gas formed within the battery.

It is a still further object of the invention to provide such a method that can be expeditiously practiced using relatively simple and inexpensive equipment.

SUMMARY OF THE INVENTION By an aspect of the invention, a method is provided for continuously producing a sheet suitable for convers'ion into a battery separator and having two principal opposite surfaces, at least one of which has a series of spaced apart projections thereon. These projections may be ribs extending transversely across the sheet. In other instances the projections may be mounds projecting upwardly from the primary surface of the sheet. In any event the process involves delivering a mass of readily deformable plastic material, preferably in self supporting sheet form having relatively plain opposite surfaces, to a calendering means where the plastic material is calender molded. Preferably, the molding takes place between two calendering rolls at least one of which has depressions formed therein that are interrupted circumferentially and spaced around the circumference of the roll. Preferably, the thus formed sheet is then sized by being subjected to pressure, preferably by being passed between two sizing rolls that have a preset sizing gap therebetween. At least one of the sizing rolls presses the projections inwardly of the sheet to set the final sheet thickness.

In the preferred operation of the invention, a very small bank of material from the extruded sheet is maintained at the nip of the calendering rolls. Preferably,

this bank is maintained at a low level. It is also generally necessary to transport the calendered sheet around at least a portion of the cicumference of the depression containing calendering roll and to partially harden the sheet. Then the partially hardened projections are withdrawn substantially straight out of the depressions to 2 prevent any substantial deformation or picking of the projection structure.

DESCRIPTION OF THE DRAWINGS FIG. 1 A diagramatic representation of a system for producing articles under the principles of the present invention. 7

FIG. 2 An enlarged, diagramatic fragmentary representation in top plan view of a battery sheet material produced according to my invention.

FIG. 3 A sectional view on line 33 of FIG. 2.

FIG. 4 An enlarged, diagramatic fragmentary representation in top plan view of a different battery separator sheet material produced according to my invention.

FIG. 5 A sectional view on line 5-5 of FIG. 4.

FIG. 6 A partial diagramatic representation similar to FIG. 1 but of a preferred specie.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 of the drawings and discussing the use of the invention which is considered to be the most readily practicable, a sheet extruding die 10 of the apparatus 11 may be seen. The apparatus 11 also includes two calendering rolls l2 and 13. A sheet 14 of plastic material is extruded by" the extruder and supplied to the nip of the calendering rolls l2 and 13. A small bank 15 of material is maintained at the nip of the rolls 12 and 13. Two sizing rolls l6 and 17 are provided downstream of the calendering rolls l2 and 13. Sizing roll 17 also serves as a transporting roll receiving the sheet 14 which passes under or across the sizing roll 16. Additional cooling rolls 18 and 19 and a wind up roll 20 may also be seen in FIG. 1.

By the application of the method of the present invention, a sheet suitable for conversion into a battery separator may be expeditiously continuously produced. A preferred form of the linear sheet 14 is shown in FIGS. 2 and 3. The sheet has two principal opposite surfaces, 25 and 26, one of which has smaller ribs (25) and the other (26) of which has a continuous series of upstanding spaced apart transverse ribs thereon. The ribs 29 extend upwardly from the base level 30. The sheet is commonly somewhat lacking in total thickness uniformity as it is separatedfrom the calendering roll 13. This is illustrated at part 27 of FIG. 3. The sheet is then sized for uniform thickness and ends up as illustrated at part 28 of FIG. 3.

Turning now to a more detailed discussion of the apparatus l l and its operation, a plastic material, preferably an oil filled plastic material such as is shown in Example 2 of U.S. Pat. No. 3,351,495, is extruded substantially as described in U.S. Pat. No. 3,351,495. The sheet is extruded in self supporting, easily deformable and relatively plain form as shown in FIG. 1. By relatively plain it is not meant to indicate that the sheet does not contain ripples and other irregularities of substantial amplitude due to serging at the die and irregular die lip construction, etc. In fact,-with the present process a considerable amount of such irregularity can be tolerated because of the specific preferred calendering procedure of my invention. I

The self supporting sheet is drawn through an air gap between the die 10 and the calendering rolls l2 and 13 synchronized surface speeds. In other words, the sur- 3 faces of the calendering rolls have the same rate of movement even though the rpm of the smaller calendering roll 12 would obviously be higher than that of the larger roll 13. The calendering rolls l2 and 13 are preferably operated at a constant gap to manufacture a sheet of close to uniform thickness.

Calendering rolls 12 and 13 both have a plurality of transverse grooves 35 or 37 separated by lands 36 or 38 spaced evenly around their circumferences. As the material in the sheet 14 passes between the calendering rolls, it is roll molded. The smoother side or surface 25 of the sheet is formed against the smoother roll 12 and the ribbed side or surface 26 is formed against the roll 13 which has the deeper grooves or channels therein.

Calendering roll 13 is normally warmabout 50 to 200F would be considered the preferred warm temperature. The other calendering roll 12 is generally hot, about 240225 to 275F would be considered hot, preferred and operable with the preferred plastic materials.

Calendering roll 12 may have heat supplied to it internally by means such as stream or hot water. Calendering roll 13 receives sensible heat from the sheet 14 and may be cooled internally with water to maintain its optimum operating temperature.

The calendered sheet is preferably transported around at least 20% of the roll 13 and is shown extending around approximately 50% of the circumference of the grooved roll 13. Roll 13 serves as a transporting roll and partially hardens and conditions the sheet, including the ribs, during its movement clue to the sheets dwell time or residence time in contact with the roll. This is necessary to provide some stiffness to the sheet and integrity in the ribs prior to their withdrawal from the molding surface. The sheet, however, is not entirely hardened or set because it is necessary to have some malability for subsequent treatment as shall be described hereinafter. By set, it is here meant to refer to establishing a relatively static state in the sheet. Set" as used here does not refer to heat set plastic, although in special instances such as plastic might be used. Instead the word set is used in this application to include the hardening of thermoplastics such as the preferred high density polyethylene material described in U.S. Pat. No. 3,351,495 as previously referred to. The heating of the roll 12 has been found useful, in preventing a leathery character and other molding defects from occurring on the side of the sheet in contact with the roll.

Turning attention now for a moment to considering how accommodations are made for the irregularities formed in the die area during extrusion as previously referred to, a very small bank 15 of the plastic material from the extruded sheet is maintained at the nip of the calendering rolls 12 and 13. For the most efficient operation of my process, without the requirement of extremely high pressures for maintaining the calendering rolls with the desired spaced apart gap, it is desirable to maintain this bank at a very low level. Thus it is important to maintain a careful balance in the thickness of the sheet extruded and the spacing of the calendering rolls 12 and 13. In a particular run the extruded sheet has an average thickness of 40 mils in the air gap, the bank plus the sheet at its highest point in the nip of the calendering rolls has an average thickness of 50 mils and the base thickness of the sheet when itpasses from the calendering rolls is an average of 10 mils with an average rib height for the larger ribs 29 of 20 mils above 1 4 the base surface 30 and for the lesser ribs 31 of 10 mils above the base surface 32.

To separate the sheet 16 from the calendering roll 13 with the least distortion of the sheets thickness uniformity it is necessary to withdraw the partially hardened ribs from the grooves one at a time, substantially straight out of the grooves in a manner not unlike disengaging gears and at a take away the same as or only very slightly greater than the speed at which the calendered sheet is transported by the grooved roll 13. 1f the takeaway is not a little faster than the transport speed of the calendering roll 13, the sheet will tend to go lax because it is not entirely hardened and may stretch slightly. This above described cooling and removal process has been found to prevent substantial deformation and picking of the rib structure before the sheet has been entirely set.

The sheet passes from the transport roll 13 to the transport roll 17 which engages the planar side of the sheet. The transporting roll 17 is preferably maintained cold and begins the final setting of the sheet immediately upon its engaging the sheet. The sheet is passed under the sizing roll 16 that has the same surface speed as the roll 17. The sheet is thus trapped in a preset sizing gap between the sizing roll 16, which is preferably hot, and the cold sizing transporting roll 17. It should be understood that the sheet is delivered between the two sizing rolls before it has entirely set or hardened and no heat is supplied to the sheet during the preferred sizing procedure except at the tip of the higher ribs 29 which aids in obtaining an even deformation and dimension.

It is important that the roll 17 be cooled for the shorter rib is engaged against it and can totally disappear if this is not done. For the same reason it is usually preferable for the roll 16 to be hot so as to aid in increasing the plastic deformation of this rib and thereby limit or restrict the amount of the flattening and enlarging of the outer ends of the smaller ribs so that the increased contact of the ribs on each side of the separator will be about the same with the respective battery plates. ln this regard it is preferable to have the small ribs or projections in contact with the roll 17 for a significant distance and therefore period of time by having the roll 16 contact the sheet and provide the sizing gap on the backside or downstream side of the roll 17. There are instances when the temperatures of the rolls may be adjusted to bring about a slightly greater amount of deformation on one side of the separator.

The sizing roll 16 impinges against the top of the ribs, one set of ribs at a time, pushing against the top of the ribs to set the final thickness of the sheet from one rib top to the opposite rib top. Of course, with rolls l6 and 17 both being driven they both actually serve to draw the sheet away from the roll 13 and move it over the transporting roll 17 which is downstream from the roll 13. The sheet is passed from the transporting roll 17 to the wind up roll 20 which draws it away from the transporting roll 17. If desired or necessary to complete the set of the sheet, the sheet may be drawn over additional cooling rolls l8 and 19 as shown in FIG. 1. Of course, other means of further cooling may be employed. For example, the sheet may be moved a long distance at room temperature to allow additional cooling time. In some instances further cooling may be unnecessary. In any event, the sheet is further cooled if necessary to complete its set.

It is preferred in the usual situation that the ribs 29 and 31 be exactly opposite each other as shown. However, in some instances it is desirable to offset the ribs 29 and 31.

Turning now to additional features of the invention, an oil filled composition seems to respond particularly well when it is oil filled to above more preferably above 65%, by volume prior to extrusion and the oil is retained in the sheet during the recited method sequence. Thus the oil filled composition is formed into a thin sheet. Then pressure is applied via the calendering rolls l2 and 13 and the sheet is deformed into projections before the sheet is set and before the sheet is leached. Thereafter and still prior to setting and leaching, the sheet is sized to a predetermined thickness by mashing or deforming the projections inwardly and toward the other side of the sheet. The thickness of the sheet is then finally set, after which the sheet is leached to remove at least part of the oil.

Many variations are possible in the application of the invention and some of them are alternate preferred species, for example the calendering rolls l2 and 13 may have a series of depressions shaped like relatively rounded pits rather than like ribs. Under these conditions the sheet of FIGS. 4 and 5 would be produced and it will be understood that the depressions in rolls 12 and 13 would be the opposite of the projections 45 and 49 on the sheet 46. As with the prior linear sheet, there is some irregularity normally present in the first calendered, molded sheet, as shown at region 47 in FIG. 5, which is corrected by the sizing rolls 16 and 17 as illustrated at 48 of FIG. 5. It is desirable, although less critical because of the more rounded shape of the projections, to withdraw the sheet from the molding calendering roll in the same manner as the previous sheet was withdrawn with the projections being pulled substantially straight out of the depressions.

It is preferable that the projections be'exactly opposite each other. However, in special situations this is not true. Furthermore, in the most preferred form of the invention theprojections are not onl y directly opposite to each other but the flattened'tips are restricted to relatively small dimensions. This enables the outer tipsof the projection to lock the plates in position by actually impinging ever so slightly into the outer surface of the battery plates stabilizing the battery plates against movement.

By a special and preferred specie of my invention that is particularly adapted to the formation of sheet having a high projection heightrib 29 in excess of 40 mils it is preferred to move the sheet over roll 17a as a cooling conveyor and than pass it to a pair of sizing rolls 16a and 1612, FIG. 6. In other respects the apparatus is the same. The process, however, further differs for the sheet is substantially finally set by the roll 17a. Final set as used in this application means that the sheet has assumed sufficient strength and dimensional stability so that further handling will generally result in predominantly elastic rather than plastic deformation. Because the highest projection tops 29 have received no prior deformation it is usually preferable to engage them against roll 16a. Sizing roll 16a preferably a sizing heating conveying roll and heats the outer ends of the tips to aid in sizing of the sheet by the downward deformation of the outer ends in the sizing nip or gap. The sizing roll 16b is cooler than roll 16a and is usually preferably cooled.

Preferably, the shorter projections are engaged against the roll 17 and the roll 17a because this generally results in less distortion of the sheet and projections. Because this results in some flattening of the projections, it is important to set the temperature on the ribs at the sizing gap to bring about the desired degree of distribution of the sizing on the respective projections. This may involve an emperical procedure of adjusting temperatures and pressures in the sizing rolls. It may also be desirable in some instances to provide a matching pattern on the sizing rolls to receive the calendered structure to more carefully size the sheet. In certain instances it is also possible to change the thickness of the finished sheet at the sizing rolls by adjusting the gaptherebetween. Thus without change, the calendering rolls l2 and 13 can be utilized to produce sheets that have slightly different end thickness requirements.

By extruding a sheet of material containing an amount of plastic averaging out in an approximation of the amount of material needed for the final sheet, an extremely high quality and uniform sheet can be produced with relatively basic and inexpensive equipment. The secondary roll sizing operation further inhances this capability. And this combined with the system for removal of the sheet from the calendering roll and the use of a highly oil filled plastic provides the ultimate in this capability. Among other things, the small bank 15 at the nip of the calendering rolls serves to minimize quality variation which could be caused by chain alignment in high molecular weight polymers.

I The process works well even with the high inorganic solids filling contained in the plastic material, which is surprising. The preferred plastic material as taught in US. Pat. No. 3,351,495 contains 5 to 60 volume percent of an inert filler material and the preferred embodiment contains 15 volume percent filler. The patent further gives dry, finely divided silica as the preferred filler. Thus the process includes operation with plastic compositions containing very substantial quantities of inert inorganic solids.

In other special situations the method of the invention maybe applied to the production of non-oil filled battery separator sheet material although it is by far most expeditiously workable with the present preferred plastic material as already stated. In particular the invention may in some instances be applied to the very highly plasticized polymer system using materials other than oil. It will be obvious to those skilled in the art that in special situations various combinations or portions of my overall invention may be used with certain steps omitted. Patterned sheets produced according to my invention may also have other special uses than as battery separators where, for example, another patterned porous membrane of similar characteristics is desired. It will thus be obvious that many changes and modifications may be made without departing from the true spirit and scope of the invention and it is, therefore, aimed in the following claims to cover all such equivalent variations as fall within the true spirit and scope of the invention.

I claim: I

1. A process for continuously producing a porous sheet membrane suitable for conversion into a battery separator and having two principal opposite surfaces, one of which has short projections thereon and the other of which has higher projections thereon comprising extruding a self supporting easily deformable relatively plain sheet of thermoplastic material, said thermoplastic material being oil filled to above 30% by volume; calender molding said sheet between two calendering rolls having synchronized surface speeds, one of said calendering rolls having shallow depressions that are interrupted circumferentially and spaced around the circumference thereof and being hot and the other of said calendering rolls having deeper depressions that are interrupted circumferentially and spaced around the circumference thereof and being warm, said sheet surface having said short projections being formed against said calendering roll having said shallow depressions and said sheet surface having said higher projections being formed on said calendering roll having said deeper depressions; forming a bank of said oil filled material from said extruded sheet at the nip of said calendering rolls and maintaining said bank at said nip; transporting the calendered sheet around at least 20% of the circumference of said calendering roll having Said deeper depressions and partially setting said calendered sheet including said projections; withdrawing said partially set higher projections from said deeper depressions, substantially straight out of said deeper depressions and at a rate of takeaway slightly greater than the speed at which said calendered sheet is transported by said calendering roll having said deeper depressions to prevent picking of said higher projection structure; before said sheet has entirely set engaging the side thereof having said short projections with a transporting roll and passing said sheet across a sizing roll that has the same surface speed as said transporting roll and trapping said sheet in a preset sizing gap between said sizing roll and said transporting roll with said sizing roll impinging against the top of said higher projections and pushing against the top of said higher projections and permanently deforming said higher projections and reducing their height and thereby setting the final thickness of said sheet; thereafter further cooling said sheet to complete its set and thereafter leaching at least enough of said oil from said sheet to make said sheet porous.

2. The method of claim 1 wherein said deeper depressions are transverse grooves and said higher projections are upstanding ribs extending sequentially transversely across said sheet, and sheet being a linear sheet, and said sizing 'roll impinging against the tops of said higher ribs one at a time.

3. The method of claim 1 wherein said shortprojections and said higher projections are formed exactly opposite each other.

4. A method for continuously producing a sheet suitable for conversion into a battery separator and having two principal opposite surfaces with series of spaced apart projections thereon comprising delivering a mass of readily deformable plastic material to the nip of two 8 opposed calendering rolls'having depressions that are interrupted circumferentially and spaced around their respective circumferences, one of said calender rolls being warm and having deeper depressions and the other of said calender rolls being hot and having shallower depressions; forming said plastic material between said two calendering rolls and in said depressions producing a sheet with projections thereof, the projections on the side of the sheet molded by said roll with said deeper depressions being higher than the projections on'the side of the sheet molded by said roll with said shallower depressions which projections are short; and passing said formed sheet between'two sizing rolls that have a preset sizing gap therebetween pressing said projections inwardly of said sheet to set the final thickness of said sheet, the sizing rollengaging the higher projections being heated.

5. The method of claim 4 wherein the short projections and the higher projections are formed opposite each other. i

6. The method of claim 5 wherein said thermoplastic material is oil filled to above 30% by volume and filled to above 5% by volume with an inert finely divided silica filler.

7. A process for continuously producinga porous sheet membrane suitable for conversion .into a battery separator and having a first surface with relatively high projections spaced apart thereon and an opposite surface with relatively shorter projections spaced apart thereon, comprising extruding a self supporting easily deformable sheet of thermoplastic material; calender molding said sheet between two calendering rolls, each of said calendering rolls having depressions that are interrupted circumferentially and spaced around the circumference thereof, one of said calender rolls being warm and having deeper depressions to mold said higher projections and the other of said calender rolls being hot and having shallower depressions to mold said shorter projections said sheet surface having said shorter projections being formed on said calendering roll having said shallower depressions and said sheet surface having said higher projections being formed on said calendering roll having said deeper depressions; transporting the calendered sheet around at least 20% of the circumference of said calender roll having said deeper depressions and partially setting said calendered sheet; removing said sheet from said calender roll having said deeper depressions and engaging the side of said sheet having said shorter projections with a cooling conveying roll; thereafter engaging said sheet between said two sizing rolls and sizing said sheet, the sizing roll engaging said higher projections being heated. 

1. A PROCESS FOR CONTINUOUSLY PRODUCING A POROUS SHEET MEMBRANE SUITABLE FOR CONVERSION INTO A BATTERY SEPARATOR AND HAVING TWO PRINCIPAL OPPOSITE SURFACES ONE OF WHICH HAS SHORT PROJECTIONS THEREON AND THE OTHER OF WHICH HAS HIGHER PROJECTIONS THEREON COMPRISING EXTRUDING A SELF SUPPORTING EASILY DEFORMABLE RELATIVELY PLAIN SHEET OF THERMOPLASTIC MATERIAL, SAID THERMOPLASTIC MATERIAL BEING OIL FILLED TO ABOVE 30% BY VOLUME CALENDER MOLDING SAID SHEET BETWEEN TWO CALENDERING ROLLS SYNCHRONIZED SURFACE SPEEDS ON OF SAID CALENDERING ROLLS HAVING SHALLOW DEPPRESSION THAT ARE INTERRUPTED CIRCUMFERENTIALLY AND SPACED AROUND THE CIRCUMFERENCE THEREOF AND BEING HOT AND THE OTHER OF SAID CALENDERING ROLLS HAVING DEEPER DEPRESSION THAT ARE INTERRUPTED CIRCUMFERENTIALLY AND SPACED AROUND THE CIRCUMFERENCE THEREOF AND BEING WARM, SAID SHEET SURFACE HAVING SAID SHORT PROJECTION BEING FORMED AGAINST SAID CALENDERING ROLL HAVING SAID SHALLOW DEPRESSIONS AND SAID SHEET SURFACE HAVING SAID HIGHER PROJECTION BEING FORMED ON SAID CALENDERING ROLL HAVING SAID DEEPER DEPRESSIONS FORMING A BANK OF SAID OIL FILLED MATERIAL FROM SAID EXTRUDED SHEET AT THE NIP OF SAID CALENDERING ROLLS AND MAINTAINING SAID BANK AT SAID NIP TRANSPORTING THE CALENDERED SHEET AROUND AT LEAST 20% OF THE CIRCUMFERENCE OF SAID CALENDERING ROLL HAVING SAID DEEPER DEPRESSIONS AND PARTIALLY SETTING SAID CALENDERED SHEET INCLUDING SAID PRROJECTING WITHDRAWING SAID PARTIALLY SET HIGHER PROJECTIONS FROM SAID DEEPER DEPRESSIONS SUBSTANTIALLY STRAIGHT OUT OF SAID DEEPER DEPRESSIONS AND AT A RATE OF TAKEAWAY SLIGHTLY GREATER THAN THE SPEED AT WHICH SAID CALENDERED SHEET IS TRANSPORTED BY SAID CALENDERING ROLL HAVING SAID DEEPER DEPRESSIONS TO PREVENT PICKING OF SAID HIGHER POJECTION STRUCTURE BEFORE SAID SHEET HAS ENTIRELY SET ENGAGING THE SIDE THEREOF HAVING SAID SHORT PROJECTIONS WITH A TRANSPORTING ROLL AND PASSING SAID SHEET ACROSS A SIZING ROLL AND THE SAME SURFACE SPEED AS SAID TRANSPORTING ROLL AND TRAPPING SAID SHEET IN A PRESET SIZING GAP BETWEEN SAID SIZING ROLL AND SAID TRANSPORTING ROLL WITH SAID SIZING ROLL IMPINGING AGAINST THE TOP OF SAID HIGH PROJECTIONS AND PUSHING AGAINST THE TOP OF SAID HIGH PROJECTIONS AND PERMANENTLY DEFORMING SAID HIGHER PROJECTION AND REDUCING THEIR HEIGHT AND THEREBY SETTING THE FINAL THICKNESS OF SAID SHEET THEREAFTER FURTHER COOLING SAID SHEET TO COMPLETE ITS SET AND THEREAFTER LEACHING AT LEAST ENOUGH OF SAID OIL FROM SAID SHEET TO MAKE SAID SHEET POROUS.
 2. The method of claim 1 wherein said deeper depressions are transverse grooves and said higher projections are upstanding ribs extending sequentially transversely across said sheet, and sheet being a linear sheet, and said sizing roll impinging against the tops of said higher ribs one at a time.
 3. The method of claim 1 wherein said short projections and said higher projections are formed exactly opposite each other.
 4. A method for continuously producing a sheet suitable for conversion into a battery separator and having two principal opposite surfaces with series of spaced apart projections thereon comprising delivering a mass of readily deformable plastic material to the nip of two opposed calendering rolls having depressions that are interrupted circumferentially and spaced around their respective circumferences, one of said calender rolls being warm and having deeper depressions and the other of said calender rolls being hot and having shallower depressions; forming said plastic material between said two calendering rolls and in said depressions producing a sheet with projections thereof, the projections on the side of the sheet molded by said roll with said deeper depressions being higher than the projections on the side of the sheet molded by said roll with said shallower depressions which projections are short; and passing said formed sheet between two sizing rolls that have a preset sizing gap therebetween pressing said projections inwardly of said sheet to set the final thickness of said sheet, the sizing roll engaging the higher projections being heated.
 5. The method of claim 4 wherein the short projections and the higher projections are formed opposite each other.
 6. The method of claim 5 wherein said thermoplastic material is oil filled to above 30% by volume and filled to above 5% by volume with an inert finely divided silica filler.
 7. A process for continuously producing a porous sheet membrane suitable for conversion into a battery separator and having a first surface with relatively high projections spaced apart thereon and an opposite surface with relatively shorter projections spaced apart thereon, comprising extruding a self supporting easily deformable sheet of thermoplastic material; calender molding said sheet between two calendering rolls, each of said calendering rolls having depressions that are interrupted circumferentially and spaced around the circumference thereof, one of said calender rolls being warm and having deeper depressions to mold said higher projections and the other of said calender rolls being hot and having shallower depressions to mold said shorter projections said sheet surface having said shorter projections being formed on said calendering roll having said shallower depressions and said sheet surface having said higher projections being formed on said calendering roll having said deeper depressions; transporting the calendered sheet around at least 20% of the circumference of said calender roll having said deeper depressions and partially setting said calendered sheet; removing said sheet from said calender roll having said deeper depressions and engaging the side of said sheet having said shorter projections with a cooling conveyiNg roll; thereafter engaging said sheet between said two sizing rolls and sizing said sheet, the sizing roll engaging said higher projections being heated. 